This invention relates to a thickness-shear mode quartz oscillator such as an AT-cut or a BT-cut.
It has experimentally been confirmed by the inventors of this invention that in thickness-shear mode quartz oscillators, there is a "frequency abnormal phenomenon" which is a different phenomenon from the well-known jump phenomenon (the phenomenon in which the main vibration of the thickness-shear made quartz oscillator is drawn into sub-vibrations such that its actual mode of vibration is different from the intended mode of the main vibration). If this "frequency abnormal phenomenon" is present within the practical application range of temperature and frequency of the use of the quartz oscillator, there take place various problems such as insufficient performance and unstable operation of the quartz oscillator and so forth.
FIGS. 1 and 2 illustrate an example of a heretofore known planoconvex type AT-cut quartz oscillator (frequency f.sub.0 =4.2 MHz). Driving electrodes 2, 2 having a diameter of 5.20 mm are formed on both surfaces of a quartz slice 1 having a diameter of 8.00 mm and a center thickness of 0.41 mm by evaporation or other suitable method. Take-out electrodes 2a, 2a are formed to extend from the driving electrodes 2, 2 in the radial direction and are connected to supporting springs (not shown). An addition mass 3 is formed on one of the driving electrodes 2 by evaporation or other suitable method in order to adjust the oscillation frequency f.sub.0. The addition mass 3 has a real circle form and its center is in conformity with that of the driving electrode 2. The addition mass is generally made of the same material as that of the driving electrode (e.g. Au, Ag, etc.). At times, addition masses 3 are fitted to both driving electrodes 2, 2. The evaporation quantity of this addition mass 3 is referred to as a "plate-back quantity" and expressed in terms of a frequency drop (KHz) caused by the addition of the same.
A number of the quartz oscillators having the above-described construction were individually driven, as a sample 4, by a driving circuit consisting of a C-MOS type integrated circuit 5 not having a frequency selection circuit in its oscillation loop, and load capacitances C.sub.1, C.sub.2 and C.sub.3 as shown in FIG. 3 to measure its frequency-temperature characteristics. Results of the measurement are shown in FIG. 4. The portions encompassed by a circle represent the positions at which the abovementioned "frequency abnormal phenomenon" occurs.
It has been found that this "frequency abnormal phenomenon" arises from the mutual interference between a fundamental wave current fed back by the fundamental main vibration and a harmonic wave current fed back by the harmonic vibrations, and is different from the conventional jump phenomenon. It has also been clarified that this phenomenon occurs with a higher probability especially in the sample having formed the addition mass 3 on the driving electrode 2 for the fine adjustment of the frequency as shown in FIGS. 1 and 2, and this tendency occurs more remarkably in convex lens type AT-cut quartz oscillators such as a planoconvex type of FIG. 5-B, a biconvex type of FIG. 5-C and bevelling types of FIGS. 5-C and 5-D than a flat plate type of FIG. 5-A. (It has been confirmed that the "frequency abnormal phenomenon" takes place also in a sample not having the addition mass film 3).
FIG. 6 illustrates the frequency-phase characteristics (resonance characteristics) of a sample of the conventional planoconvex type AT-cut quartz oscillator (plate-back quantity of the addition mass 3=4 KHz) where f.sub.0 is a fundamental frequency and f.sub.31 and f.sub.32 are a group of harmonics that are adjacently present near the triple frequency of the fundamental frequency f.sub.0. As shown in the load capacitance characteristic curve of FIG. 7, the fundamental frequency f.sub.0 varies in accordance with the load capacitance C.sub.L. (The frequency adjustment of the quartz oscillator uses this phenomenon). However, f.sub.31 /3 and f.sub.32 /3 hardly cause any change. In the practical application, the load capacitance C.sub.L is mostly set to 10 PF or below because a smaller load capacitance generally provides a wider range for the frequency adjustment. Accordingly, in the practical application range of the load capacitance C.sub.L, it sometimes occurs that f.sub.0 coincides with f.sub.32 /3. Even when they do not, they become coincident with each other depending upon certain temperature conditions employed, because they are extremely approximate to each other. In this instance, f.sub.32 /3 exerts an influence on f.sub.0 and thus causes the "frequency abnormal phenomenon". This generally applies to a group of harmonics of a multiple of an odd number of the fundamental frequency f.sub.0.
In other words, in the AT-cut quartz oscillator, there are always present a series of groups of harmonics in the range of frequency n-folds (n=odd number) in which the fundamental frequency f.sub.0 and a harmonic frequency f.sub.ns satisfying the following equation (1) is present with high probability in accordance with the production condition of the quartz oscillator, the driving circuit used and other factors; EQU f.sub.0 =f.sub.ns /n (1)
For the abovementioned reason, the "frequency abnormal phenomenon" shown in FIG. 4 occurs with high probability. The magnitude H of the "frequency abnormal phenomenon" (see FIG. 4) becomes greater when the degree n of the harmonic wave f.sub.ns is smaller.
The reason why this "frequency abnormal phenomenon" has not so far been a serious problem is that the occurrence of the phenomenon depends to a large extent upon the kind and performance of the driving circuit used to drive the quartz oscillator. This "frequency abnormal phenomenon" does not occur, for example, when the AT-cut quartz oscillator is driven by an oscillator circuit having a frequency selection/tuning circuit such as a tank circuit. Also, even when a C-MOS type integrated circuit 5 not having a frequency selection circuit in its oscillation loop (as shown in FIG. 3) is used, the "frequency abnormal phenomenon" does not occur, or is small even when its does occur, if the high frequency response characteristics of the integrated circuit 5 are not very good. As the high frequency response characteristics of the integrated circuit 5 become better, however, the "frequency abnormal phenomenon" occurs more remarkably with higher probability. Hence, the "frequency abnormal phenomenon" will become a serious problem in the near future as the development of the integrated circuit further proceeds and its high frequency response characteristics become more enhanced.